Orographic Banner Clouds 2. EULAG Workshop Björn Brötz, Daniel Reinert, Volkmar Wirth Institute for Atmospheric Physics/ University of Mainz 2 nd EULAG Workshop

2. EULAG Workshop Occurrence: Banner clouds are observed at steep 3D mountain summits or on quasi 2D mountain ridges Characteristic property: The cloud exists only in the leeward side The windward side is cloud free Rysy, Poland Matterhorn, Switzerland

Definition of the banner cloud Mechanism of formation Previous work / Simulations with EULAG Sensitivity on the thermodynamic conditions Summary 2. EULAG Workshop Outline

Quasi stationary phenomenon Lifetime up to several hours Strong turbulence Conic shape, strongest vertical extent close to the obstacle Typical horizontal scale about 1 km Frequent occurrence (2-3 times per month at mount Zugspitze) 2. EULAG Workshop Definition Matterhorn, Switzerland

1.Bernoulli-Theory: Formation due to reduction of pressure (adiabatic expansion) in the area of strong acceleration close to the summit (Beer, 1974) 2.Fog-mixing-Theory: The mixing of two differently saturated air masses with different temperatures and specific humidity (Humphreys, 1964) 3.Lee-rotor-Theory: The banner cloud is the visible result of forced ascent in the upwelling part of the lee rotor (Glickman, 2000) 2. EULAG Workshop Mechanism behind

Messungen am Zugspitzgrat (2005) Bannercloud 2. EULAG Workshop Meteorological conditions Leeward air is 3-4K warmer and 40-70% more humid then the cloud free air on the windward side. i.e. locally very strong instable stratification; even so the appearance of the cloud is stable and persistent

2. EULAG Workshop Previous work Area of lifting Staupunkt Reinert, D., and V. Wirth, 2008

2. EULAG Workshop Simulations with EULAG Significant ascent in the lee Strong asymmetry between windward and leeward flow field Larger vertical extent of ascending region in the lee ward side

2. EULAG Workshop That supports the lee rotor theory In order to gain information about the needed thermodynamic profiles lagrangian information is important

2. EULAG Workshop Initializing of a passive tracer z x zszs Δz: mean vertical displacement zszs Advection of a passive tracer Initialized by its altitude at t 0

2. EULAG Workshop Mean vertical displacement, m Strong windward leeward asymmetry Strongest positive vertical displacement in in lee  Different air masses or additional sources of humidity are not necessary Similarity of the tracer field to the banner cloud

2. EULAG Workshop Thermodynamic conditions Thermodynamic PropertiesDynamic Properties LCL(z s ) Vertical profile of the Lifting Condensation Level as a function of starting height z s. Vertical profile of the mean maximum height of air parcels in the very near of the leeward/windward side as a function of the starting height z s. 1 profile1 profile for the leeward and 1 profile for the windward side

2. EULAG Workshop Diagram shows Situation that fulfills the conditions Implications for the profiles of θ, q v, θ e LCL-Profile cannot be constant in height; needs to increase with height Very favorable are the situations with: LCL Conditions: with LCL LCL

Mean environmental profile for u Satisfy the criterions Initial conditions of the simulations Mean environmental profiles for θ, q v, θ e moist stable Structure of the profiles motivated by measurements at mount Zugspitze during a banner cloud event Precise form was adapted to match the defined criterions LCL

Simulated banner cloud Unfortunately the banner cloud simulated by EULAG is still not available… but coming soon

Simulated banner cloud of previous study Iso-surface of the specific cloud water content q c, colors show the instantaneous vertical velocities Draufsicht

Sensitivity of the cloud on the LCL profile Conditions: with LCL LCL

Sensitivity of the cloud on the LCL profile Mittlerer Wolkenwassergehalt

Simulation compared to measurements Messungen am Zugspitzgrat (2005) Bannercloud Typical observation for banner clouds: higher values for T, q tot (and so θ e ) in the lee

Simulation compared to measurements Time series of θ e, q tot, T for simulated Banner cloud Higher values of T, q tot (θ e ) in the leeward side in the simulation. This is the result of the differential lifting and the gradients in the imposed profiles of T and q tot Lee

Recent research of our group strengthened the lee-rotor-theory as the mechanism behind the formation of the banner cloud. (dynamic phenomenon) The thermodynamic conditions had been identified for the case of a steep pyramidical shaped mountain EULAG reproduced some of the results of previous simulations that give confidence if the previous results Summary

After the successful simulation of the banner cloud with EULAG the next steps will be Investigation of the banner clouds on quasi 2D mountain ridges (like mount Zugspitze) Use of real topography Use of trajectories instead of the passive tracer Investigations on the sensitivity of the static stability of the Flow. Is there a regime a < Fr < b beyond the banner cloud cannot occur (e.g. due to the missing lee rotor) Outlook

Thank you 2. EULAG Workshop